We present an ultra-thin transmission-type radiation detector developed for counting microbeam alpha particles. The ∆E alpha detector is a single crystal chemical vapor deposited diamond (ScCVDD) and will be installed between the microbeam accelerator window and a biologic sample. The commercially available optical grade ScCVDD sample (3 mm × 3 mm × 50 µm) was etched down to a few µm thickness which allows alpha particles to penetrate, and then it was followed by the surface cleaning, electrical contact deposition and post-metallization annealing. SRIM code and MCNP6 were used for energy loss calculation of alpha particles in electrodes and diamond and pulse height spectra prediction. In order to evaluate the performance of the ultra-thin ScCVDD detector, a ∆E-E detectors system was setup using a calibration source, the ScCVD detector and a silicon surface barrier detector (SBD). The absolute and intrinsic totally efficiency were determined as 0.3 % and 16 % respectively. Alpha and gamma peaks were observed while the peak resolution is not quite promised. The transmission ability of the ScCVDD detector was verified by applying coincidence operation with 0.22 µs time window. The thickness of the ultra-thin diamond sample was reassessed to be 8.315±0.690 µm from ∆E-E spectrometery. / Thesis / Master of Science (MSc)